Some of those older machines had low air warning lights/buzzers that quit working, assuming they had them at all. If there's an air gauge, watch it build pressure. Could take a couple minutes depending on how neglected that air system has been. Can't tell you what the release pressure would be but my best guess is that you're pushing to release the brakes but they're not actually releasing because of lack of pressure.

Better yet, call the pit and have them square you out seeing as how you're already doing them a favor by loading your own material. Least they could do is walk you through running their gear, that ancient stuff has quirks without a doubt.